Geology of the Great Basin

10. The Paleozoic

10 the paleozoic passive subsidence Slowly, ponderously, the former piece of North America moved westerly. This huge cratonic fragment may have drifted across almost one-half of the globe before affixing itself onto the continental margin of Asia. Later fragments were glued onto its outboard margin, embedding the misplaced fragment of America deeply into Siberian exile. Meanwhile, back along the newly rifted border of North America, the thermal welt also moved westerly away from our shore. Our margin cooled, contracted, and subsided. Just as a cooling oven loses the most heat in the first few minutes after it is turned off and slowly loses the remainder, so it is when magmatic heat is turned off. The initial heat loss, after the thermal spreading center moved away from the continent, was great, and consequently the contraction and sinking were greatest in the first years of cooling. Slowly, equilibrium was reestablished, and the rate of thermal contraction slowed. Data from a more recent rifting episode in the Atlantic Ocean indicate that contraction episodes require about fifty million years. The uplifted Prepaleozoic continent was barren. There were no land plants to hold the weathered soils, nor were there any land animals to witness the erosion of the highlands. The life in the sea was simple and so soft as to leave almost no record. Some glacial deposits from six hundred to seven hundred million years ago have been described in the western United States. Perhaps the earth was considerably cooler due to an elliptical orbit around the sun. Or maybe the highlands were elevated far enough to catch snow. It Devil’s Gate, central Nevada. A mountain range resulted from the accretion of plates in the central Great Basin 350 million years ago. Today, only the fragments of erosion from this range remain. Beds of this debris outcrop at Devil’s Gate near Eureka, Nevada. Tom Brownold. The Great Basin during the early Paleozoic. The Paleozoic 95 has also been proposed that rapid drifting may have brought the glaciated areas into the high latitudes during this one hundred-million-year time period, although this seems doubtful. A great flood of sediments cascaded down the desolate flanks of the thermally swollen mountainous borderland and poured into the marginal sea. The dual forces of thermal contraction and erosion slowly lowered the continental highland. As the mountains were reduced, the erosive productions decreased, and the sediments piling up in the ocean along the continental border decreased in volume. The mountains were reduced to rubble, and the continental margin was buried by a huge pile of debris. What had been a lofty highland was now a great westward-thickening prism of Prepaleozoic sediments. This was the beginning of the Cordilleran trough, a slowly subsiding and sediment-filled basin adjacent to the continent. The clastics blanketed the near-shore and continental-shelf environments. Fragments of the eroded continent were swept by ocean currents as far west as central Nevada. As thermal contraction continued, the continent sank beneath the sea. The ocean invaded easterly, across the subsiding crust, and shallow waters encroached over the continental domain. By Early Cambrian time as much as twenty thousand feet of clastic and carbonate sediments were deposited along this continental shelf along the edge of the craton. The depositional environments from the late Prepaleozoic through the early Paleozoic were essentially the same. The rocks, therefore, are very similar. Geologists will stand on an outcrop and argue for hours, trying to determine the contact between the Prepaleozoic and the Paleozoic, unless there is an abundance of unequivocal fossils. The Cambrian sea was shallow in the eastern Great Basin. Along the shoreline we see today, mud cracks indicate periods of drying above sea level. Some of the muds have the distinct imprints of raindrops. Imagine holding a rock in which the impressions of an antediluvian deluge are still preserved after almost six hundred million years! Even blasé geologists, often inured by constant exposure to the vast geologic time span encapsulated in the rocks around them, can’t help but be impressed by the delicate preservation of such a fleeting moment of time preserved in a rock at least ten million times their age. The delicate tracings of the impressions of algae, which require sunlight for survival and hence live in shallow, well-lit waters, are also found in these rocks. Movement of sediments by ancient tidal currents is indicated by the layering and...